The invention set forth in this specification pertains to both methods and apparatuses for processing metal scrap so as to recover the metal in such scrap. Both of the methods and apparatuses indicated in this specification are considered to be particularly adapted for use in connection with the recovery of various metals and alloys from various other types of scrap.
It is believed it is well recognized that it is desirable to process aluminum scrap so as to recover the metal aluminum so that this metal can be reutilized. Scrap aluminum is normally contaminated with two different types of contaminants--organic contaminants and inorganic contaminants. The organic contaminants most commonly consist of remnants of various types of oils, remnants of various types of coatings or paints and the like. The latter may or may not contain significant amounts of inorganic pigments. The inorganic contaminants present may include dust particles, pigments as indicated in the preceding, some minor amounts of various scrap metals (other than the principal metal within the scrap) and the like. Aluminum scrap will also normally contain varying amounts of aluminum oxide resulting from the oxidation of the aluminum scrap and/or from appropriate anodizing procedures employed in processing the metal from which the scrap originated.
Both the organic and inorganic contaminants as indicated in the preceding are preferably removed to as great a degree as possible from the scrap prior to the scrap being melted down in an appropriate furnace or melter in order to avoid interference with the operation of the furnace or melter and in order to minimize to as great a degree as reasonably possible the chances of the molten metal obtained from the furnace or melter being contaminated. It has been recognized that an effective manner of getting rid of the organic contaminants present on a scrap metal such as scrap aluminum is to heat the scrap aluminum to a sufficient temperature so that substantially all of the organic contaminants will decompose and so that the resulting decomposition products will substantially all vaporize.
It has also been recognized preferably such scrap should be heated at a temperature which is sufficiently low as to minimize oxidation of the aluminum and at a temperature which is sufficiently low so that there is no reasonable chance of the aluminum scrap tending to agglomerate or fuse into a body which is difficult to handle or which has to be broken up. Further, it has been recognized that the amount of time that the scrap is heated should be controlled so that the scrap is heated no longer than is reasonably necessary to decompose the organic contaminants and is not held at an elevated temperature sufficient to accomplish such decomposition for a sufficient period for agglomeration of the aluminum particles to take place.
Although a wide variety of different separation techniques based upon difference in various physical properties have been capable of being used to recover inorganic contaminants from metal scrap such as aluminum scrap, it normally has not been economic to utilize such procedures. In the recovery of aluminum from aluminum scrap various inorganic contaminants of an oxide character have normally been separated in a furnace or melter as indicated. The majority of such contaminants will float to the top of a bath of molten aluminum to form slag or slag-like skin of inorganic contaminants on the molten metal which can be skimmed off of the metal in accordance with well established techniques. However, excessive slag formation during the recovery of metal such as aluminum is disadvantageous because of the labor costs in removing the slag and the fact that some of the molten metal is normally removed from a furnace along with the slag.
Several different procedures have been proposed and to various extents adopted for the removal of organic contaminants from scrap such as aluminum scrap. One method has involved heating a bed or body of scrap either directly or indirectly to a point where the various organic contaminants vaporize so that they can be ignited. Other procedures have involved conveying a bed of aluminum through a heated chamber while either hot gas is circulated through the bed or while a flame is directed toward the moving bed of scrap.
All of these procedures are considered to be disadvantageous for any one of a variety of different reasons. Whenever a flame is directed at a bed of aluminum scrap there is a significant probability of at least some of the aluminum being oxidized. Of course any such oxidation results in a lowering of the amount of aluminum metal recovered. Further, the heating of aluminum scrap is relatively difficult to control because the quantity of organic contamination of such scrap may vary significantly. There have been instances where the procedures employed for removing organic contaminants have been relatively unsafe as a result of explosion hazards caused by the presence of significant quantities of organic material in air.
It is considered that the economic desirability of recovering metal from metal scrap coupled with the limitations of the processes as are indicated in the preceding have stimulated interest in improved manners of recovering metal from scrap. Such current work as has been done in this field has tended to recognize that improvement in metal recovery from scrap has required a complete "system" in which individual operations are interrelated.
As an illustration of this it has recently been recognized that aluminum can be recovered from aluminum scrap by using a process in which aluminum scrap is fed into the upper inlet end of a rotary kiln located so that the discharge end of the kiln discharges the scrap directly into a melting furnace. In this process the kiln and the furnace are connected by appropriate conduits or ducting containing a burner and a blower so that there is a continuous gas flow through the furnace and then through the kiln. This flow is counter-current to the direction of scrap flow in the kiln. The burner serves to maintain the temperature of the recycled gas to a desired valve. With this type of system some of the recycled gas is bled off from the system through a vent in the furnace so it can be discharged to the atmosphere. This separated gas may be passed through a recuperator so as to preheat either air supplied to the burner to sustain combustion or the fuel burned in the burner or both.
While this type of process is considered to be advantageous as compared to prior procedures it is considered to be disadvantageous for several reasons. The rotary kiln used with this procedure is essentially operated in a conventional manner so as to heat the scrap passing through it by the counter-current flowing gas stream. This is not considered to tend to effect any significant removal of inorganic contaminants in the scrap and, further, if there is any removal of inorganic contaminants from the scrap it is considered this procedure will only convey such inorganic contaminants back to the furnace where they will tend to settle out so as to form a slag or skin on the molten metal within the furnace. Further, because of the counter-current flow within the rotary kiln and the relationship of the kiln to the furnace the temperature of the scrap discharged to the furnace will be related to the temperature within the furnace. As a consequence of this it is considered that it is impossible to obtain the degree of temperature control in the furnace necessary for most effective removal of organic material from the scrap.
As a result of the limitations of prior processes for treating metal scrap it is considered there is a need for improving the economics of such treatment. It is particularly considered that there is a need to improve upon the treatment of metal scrap so as to effectively utilize heat which is essentially waste heat in prior related processes. It is also considered there is a need for an improved method of and an improved equipment for treating metal scrap such as aluminum metal scrap in such a manner as to maximize the recovery of metal from such scrap utilizing a minimum amount of fuel and labor.